This invention offers a method and the relative equipment for the drawing and dual orientation of multiple-layer thermoplastic materials, in particular of co-extruded multiple-layer materials, that allows achieving the maximum orientation ratio for all layers, even where these exhibit different drawing ratios and orienting temperatures.
In particular, the method according to the invention brings a multiple-layer co-extruded tubular exiting the preheating tunnel to the orienting temperature of the material at the highest drawing temperature, performs a first dual orienting phase while the remaining materials are still in a plastic state, drops the temperature of the tubular to the temperature of dual orientation of the second material, performs an orienting phase of the second material while at the same time completing the dual orientation of the first material, and so on for all other layers.
The method according to the invention therefore allows achieving a dual orientation of the various layers at the maxim allowable ratio for each of them, while enhancing their chemical and physical characteristics and also achieving substantial material savings.
The invention refers to the sector of producing multiple-layer extruded materials, and in particular of producing tubulars or films made of a multiple-layer thermoplastic material.
These tubulars or films are used for instance for the packaging of foodstuffs, of a pre-cooked or raw type, thanks to their low cost and chemical-physical characteristics, which ensure the favorable conservation of the product for periods of reasonable length.
These tubulars or films are generally constituted of multiple layers of materials chosen depending on their characteristics.
In particular, the material composing the inner layer must be compatible with the packaged product and be totally non-toxic, and the subsequent layers must provide an effective barrier against gases, liquids, and odors.
These tubulars or films are commonly produced by extruding layers of concentric materials through an annular die, at whose outlet the tubular is calibrated and cooled, then heated by passing it through a tunnel and subsequently dually oriented by simultaneously drawing it in two directions, a transversal and a longitudinal one.
The longitudinal draw is performed by drawing the tubular between two fittings moving at different speeds and at a speed increase between them equal to the drawing ratio, while the transversal draw is performed by the so-called xe2x80x9cbubblexe2x80x9d system, which consists in blowing a volume of air into the tubular capable of causing an increase of its diameter equal to the drawing ratio, thus generating a sort of air bubble inside the tubular stretch between the two fittings.
These drawing systems are well known to the experts of the trade, so that a detail description is unnecessary.
The different materials composing the various layers generally exhibit different fusion and orientation temperatures, while their maximum obtainable drawing ratios also vary from one material to another.
The following table outlines the mentioned parameters for the materials most commonly used in producing these tubulars:
In order to prevent the rupturing of the material composing one or more of these layers, it is obviously necessary that the drawing of the tubular to be oriented observe the values for the material having the lowest drawing ratio.
It follows, therefore, that many of the materials are not fully exploited, as despite being capable of supporting drawing ratios far higher than those applied, they are actually drawn to a lesser extent, at a drawing ratio equal to that of the material tolerating the least degree of elongation.
These limits are now exceeded by using the method according to the invention, which allows reaching the maximum possible drawing value for each layer. For this purpose the invention provides for taking the tubular exiting the tunnel up to the temperature of the material having the highest drawing temperature and to expand the tubular to start the drawing of this first layer, while the material of the remaining layers, which are still in a molten state, plastically deform by following the elongations of the first layer.
The temperature of the tubular is then reduced and controlled down to the orienting temperature of the second material, continuing to draw the first layer while at the same time drawing this second layer.
The process is thus continued until all layers are fully drawn.